It is well known to those skilled in the art that a vehicle powertrain is based notably on a crankshaft, to which there is coupled a crankshaft wheel, and a camshaft, to which there is coupled to a camshaft wheel. The crankshaft converts the rectilinear movement of the pistons into a rotational movement that drives the driveshaft. The camshaft, as its name suggests, is fitted with cams and its purpose is to convert the rotational movement of the driveshaft, imparted by the crankshaft, into a reciprocating movement either of translation, for example in the case of valves, or of rotation, for example in the case of rockers.
In this context, one of the functions of the engine management system of a combustion engine vehicle is to manage the injection of gasoline and the ignition of the engine. Present-day combustion engines comprise several cylinders and their principle of operation is well known to those skilled in the art. The engine management system needs to allow the correct quantity of gasoline to be injected at the correct moment into the correct cylinder in order to optimize the flexibility and power of the engine and minimize the resultant pollution. For this purpose it is necessary to know precisely the angular position of the driveshaft so as to ensure that the injection of gasoline is made into the cylinder that is in the intake phase. This whole process of determining which cylinder is in the intake phase, and into which the injection of gasoline is to be made, corresponds to what is referred to as the engine synchronization.
Thus, in order to ensure adequate engine management, it is absolutely essential to know the state of rotation of the engine. This state may adopt three status values:                i) either the engine is rotating, when it is turning over at a speed comprised within its operating range, namely higher than the minimum speed tolerated by the engine, below which engine management is no longer possible; in which case it is “rotating”;        ii) or the engine is not turning over at all, in which case it is “stopped”;        iii) or the state of rotation is indeterminate because it cannot be determined either that it is “rotating” or that it is “stopped”; it is therefore “in the process of stalling”. It should be noted that the expression “in the process of stalling” means an engine which, because of its low inertia, behaves very unpredictably, with rotation forward or backward, a high degree of acyclism, the possibility of suddenly stopping, etc. This “in the process of stalling” rotation state is reached, in particular, when the engine speed reaches a stalling threshold from and below which the engine finds itself in a state in which its behavior therefore becomes unpredictable.        
In order to know the state of rotation of the engine, it is known practice to consider the state of rotation of the crankshaft wheel, which is also that of the crankshaft, and the state of rotation of the camshaft wheel, which is also that of the camshaft.
Specifically, for reliability reasons, the determination that the engine has stopped rotating is based on verifying the two items of information that the crankshaft has stopped and that the camshaft has stopped.
As is known to those skilled in the art, the state of rotation of the crankshaft can be determined by observing the crankshaft wheel. The crankshaft wheel is a wheel the chief function of which is to make it possible to determine the angular position of the crankshaft. For this purpose, the crankshaft wheel comprises a plurality of teeth, numbered from a reference corresponding for example to the absence of at least one tooth on the periphery of the wheel. Typically, a crankshaft wheel of a vehicle engine comprises 60 teeth distributed about its periphery, and a gap of two missing teeth. The gap of two missing teeth serves as a reference for determining the angular position of the crankshaft wheel. Suitable detection means, positioned near the crankshaft wheel, detect the passage of the fronts of the crankshaft wheel teeth, it being understood that said detection means are configured to detect either only the rising fronts or only the falling fronts.
The state of rotation of the crankshaft is therefore determined according to the time elapsed since the last detection of a crankshaft wheel tooth-front.
The principle is therefore that if the detection means see crankshaft wheel tooth-fronts filing past, then the crankshaft is “rotating”. When the detection means no longer see crankshaft wheel tooth-fronts filing past, then the crankshaft is “stopped”. Between these two states, when the time since the last detection of a tooth-front lengthens, the crankshaft is “in the process of stalling”, the engine being in the process of stalling.
In practice, engines have constraints relating to engine speed, so as to ensure that they operate normally. Outside of a range of normal values, engine management becomes inoperative. It is considered that the information derived from the detection means used for assessing the state of rotation of the crankshaft is no longer reliable. In particular for any engine, a minimum engine speed tolerated by the engine is generally defined, the minimum engine speed being defined as the engine speed below which the engine finds itself under operating conditions in which its behavior becomes completely unpredictable and in which it is no longer possible to provide effective engine management; this speed corresponds to the stalling threshold, according to the definition given hereinabove of the “in the process of stalling” state of rotation.
The criterion for verifying the state of rotation of the crankshaft therefore consists, according to the prior art, in evaluating the time elapsed since the last detection of a crankshaft wheel tooth-front. As long as this time is shorter than the theoretical time Tvil_cal taken by the crankshaft wheel to cover, at a speed corresponding to the minimum tolerated engine speed, an angular distance corresponding to the angular distance between two successive crankshaft wheel tooth-fronts capable of being detected by the detection means, the crankshaft is considered to be “rotating”. As soon as the time elapsed since the last detection of a crankshaft wheel tooth-front becomes longer than Tvil_cal, the crankshaft is considered to be “in the process of stalling”. When the same length of time becomes longer than a length of time Tvil_stop, that has been subject to calibration, the crankshaft is determined to be “stopped”. Thus, Tvil_cal is a development value that makes it possible to suspect imminent stoppage of the engine, manifested in the “in the process of stalling” state of rotation, in this instance, of the crankshaft, and unpredictable engine behavior, whereas Tvil_stop is a development value which makes it possible to be certain that the crankshaft is completely stopped.
In general, when it is not possible to determine whether the crankshaft is “rotating” or “stopped”, then said crankshaft is considered to be “in the process of stalling”.
By way of illustration, if we consider a crankshaft wheel provided with 60 teeth evenly distributed about its periphery, then the angular separation between two successive tooth-fronts is 6°. At a minimum tolerated engine speed of 22 rpm, Tvil_cal is equal to 0.045 sec. So Tvil_stop is typically of the order of 0.3 sec.
The state machine depicted in FIG. 1 shows a state machine representing the various states of rotation of the crankshaft, as determined by the known method described hereinabove.
As is known, in order to ensure that the information relating to the state of rotation of the crankshaft is correct and that said state of rotation makes it possible to determine the state of operation of the engine, it is necessary to verify the rotation state “proposition” determined from monitoring the filing-past of the crankshaft wheel teeth by determining the state of rotation of the camshaft.
This is because the state of rotation of the crankshaft may be determined erroneously, for a number of reasons: broken crankshaft wheel tooth, malfunction of the crankshaft wheel tooth-front detection means, etc.
The camshaft is rotationally driven by a pinion which transmits the rotational movement of the crankshaft to it. Furthermore, the camshaft has cams, which allow the rotational movement of the drive shaft to be converted into a reciprocating movement, thus operating the valves. The camshaft also comprises a camshaft wheel the purpose of which is to allow the angular position of the camshaft to be determined.
However, it is possible, using means similar to those used for the crankshaft wheel, namely using a sensor that detects the passage of the camshaft wheel tooth-fronts, to attempt to determine whether the camshaft is “rotating” or “stopped”. According to the prior art, the principle making it possible to determine the state of rotation of the camshaft consists, as it did in the case of the crankshaft, in evaluating the time elapsed since the last detection of a camshaft wheel tooth-front. As long as this time is shorter than the theoretical time Tcam taken by the camshaft wheel to cover, at a speed corresponding to the minimum tolerated engine speed, an angular distance corresponding to the maximum angular distance between two camshaft wheel tooth-fronts capable of being detected by the sensor provided for this purpose, the camshaft is considered to be “rotating”. As soon as the time elapsed since the last detection of a camshaft wheel tooth-front becomes longer than Tcam, the camshaft is considered to be “stopped”.
When it is not possible to determine whether the camshaft is “rotating” or “stopped”, then said camshaft is considered to be “in the process of stalling”. It should, however, be noted that, in the prior art, the state of rotation of the camshaft, according to the state machine depicted in FIG. 1, can pass from the “stopped” state to the “in the process of stalling” state after detecting a first camshaft wheel tooth-front and before detecting a second tooth-front and then pass to the “rotating” state after detecting the second camshaft wheel tooth-front. In the other direction, starting from the “rotating” state of rotation, the camshaft wheel can pass only into the “stopped” state when the time elapsed since the last detection of a camshaft wheel tooth-front exceeds Tcam.
If it is determined that the crankshaft and the camshaft are “rotating” or “stopped”, there is no doubt that it can be deduced that the engine itself is, respectively, “rotating” or “stopped”.
However, one shortcoming of this known technique is that the camshaft wheel has very few teeth, typically only four. As a result, two successive tooth-fronts are separated by a significant degree of angle that the camshaft wheel covers, at a rotational speed that corresponds to the minimum engine speed tolerated by the engine, in a fairly lengthy amount of time.
Thus, the time taken to detect that the camshaft has stopped rotating is very long, whatever the operating conditions of the engine.
By way of illustration, at the minimum engine speed 22 rpm, for a standard camshaft, the longest tooth of which measures 146°, Tcam is equal to 1.11 sec. For this calculation, the angular distances between CAM fronts are expressed in ° CRK. Therefore, the angular distance of the CAM target is 720° CRK, whereas physically it is only 360°.
There is therefore a need for a method for determining the state of rotation of the camshaft in a reduced space of time, at least under particular engine operating conditions.
In this context, the present invention is aimed at a method making it possible to determine the state of rotation of the camshaft, under certain conditions, in a time that is far shorter than in the prior art.